The Influence of Diabatic Heating on the Development of Two North American Jet Superposition Events

The instantaneous locations of the polar and subtropical jet streams are closely tied to the pole-to-equator tropopause structure. The polar jet stream resides in the break between the polar and the subtropical tropopause and is positioned atop the strongly baroclinic, tropospheric-deep polar front at ~50°N. The subtropical jet stream resides in the break between the subtropical and the tropical tropopause and is situated on the poleward edge of the Hadley cell at ~30°N. On occasion, the latitudinal separation between the polar and the subtropical jets vanishes, resulting in the formation of a two-step pole-to-equator tropopause structure and a jet superposition. Jet superpositions are also characterized by a marked acceleration of upper-tropospheric wind speeds, a consolidation of the pole-to-equator baroclinicity into a narrow latitudinal band, and weak static stability on the equatorward side of the jet. Consequently, the development of a jet superposition typifies a dynamical and thermodynamic environment that can be particularly conducive to the development of high-impact weather.

Prior work suggests that diabatic heating often plays a critical role in the facilitating the development of the two-step pole-to-equator tropopause structure associated with a jet superposition, particularly for those superpositions that develop near the climatological latitude of the polar jet. This study quantifies the impact of diabatic heating during the development of two notable wintertime jet superpositions over southeastern Canada on 22 December 2013 and 20 February 2019, respectively. Both of these jet superpositions exhibited anomalously strong upper-tropospheric wind speeds in excess of 100 m s^–1, and were characterized by surface cyclogenesis, extensive precipitation, and strong upper-tropospheric divergent winds beneath the equatorward jet-entrance region. Two companion 30-km simulations are performed for each case using version 3.9.1 of the Weather Research and Forecasting (WRF) model, one with full physics and one without diabatic heating. Momentum budgets are subsequently calculated within each simulation to illuminate the role that three-dimensional divergent circulations play in accelerating upper-tropospheric wind speeds in the vicinity of each jet superposition, and to quantify the impact of diabatic heating on the resultant structure of the upper-tropospheric jet. Predicated on the results from these simulations, the importance of accurately representing diabatic heating within forecasts of jet superposition events is discussed.